Bi-Directional Ring Network Having Minimum Spare Bandwidth Allocation And Corresponding Connection Admission Control

ABSTRACT

The present invention provides for a method for reserving spare bandwidth for a link in a communication network including a plurality of links. The method provides for monitoring the volume of traffic routed through each link of the communication network. A single link failure for each link is then simulated and the volume of traffic which would be rerouted through each link for maintaining communication and the volume of traffic removed from each link are determined for each simulated single link failure. The difference between the volume of traffic which would need to be rerouted through each link and the corresponding volume of traffic removed from each link is then computed, and a maximum difference value is determined for each link for all simulated single link failures. An amount of spare bandwidth equivalent to the determined maximum difference is then reserved for each link.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority from provisional U.S. PatentApplication Ser. No. 60/087,264, titled “Bi-directional Ring Networkhaving Minimum Spare Bandwidth Allocation and Corresponding ConnectionAdmission Control”, filed May 29, 1998; which is hereby incorporatedherein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION

Communications networks, such as B-ISDN networks, may support largevolumes of traffic and offer a wide variety of services. The everincreasing traffic loads and the growing reliance on thetelecommunications infrastructure for both business and personalcommunication necessitate reliable networks. In connection orientednetworks, fast connection restoration after failure is a crucial elementof reliability. Self-healing methods, which automatically restorenetwork connections after failure, exist for general networkarchitectures and for ring networks. These methods typically rely ondistributed control to insure fast fault recovery and to protect againstcatastrophic failure.

Self-Healing Rings (SHRs) have proven to be an effective architecturefor delivering protected SONET service. This architecture consists of 2-or 4-fiber rings which give the ability to carry traffic in both theclockwise and the counter-clockwise direction. When a failure occurs inthe ring, traffic is switched away from the failed ring segment. SHRsoffer fast restoration after failure, 100% traffic recovery from singlelocation failures and a simple add/drop multiplexer architecture fornetwork access. SHRs rely on a type of self-healing called protectionswitching where a failed connection is automatically switched to apre-established back-up connection. End-to-end path protection switchingis used in SONET Dual-Fed Unidirectional Path Switched Rings (UPSRs).Bidirectional Line Switched Rings (BLSRs) use point-to-point lineprotection switching. In addition to SONET transport, SHRs are proposedfor other connection oriented networks such as all-optical wavelengthdivision multiplexed (WDM) networks and ATM LANs. The protectionswitching mechanisms developed for SONET SHRs are being adapted to othernetworks and network layers. For example, ATM layer protection schemesare proposed for SONET Rings carrying ATM traffic and for ATM LANs.

Self-healing protocols usually involve four steps: spare capacityallocation, failure detection, failure notification and protectionswitching. One of the critical issue's in determining the feasibility ofa SHR protection mechanism is the required capacity needed to provide100% restoration after single location failures. The ring capacityrequirement depends on the spare bandwidth allocation, the trafficdemand pattern, the protection scheme and the routing method.Traditionally, SONET connections are bidirectional and symmetric; inother words, a SONET link between two points in the network contains thesame bandwidth allocation in both directions. Standards are beingdeveloped which allow SONET connections to be unidirectional orbidirectional asymmetric. Asymmetric connections contain differentbandwidth allocations for each direction of a duplex path. Sinceasymmetric connections are possible in ATM as well, the traffic demandpatterns in future SHRs may contain asymmetric demand between nodepairs.

Three distinct methods of protection switching have been identified forring networks. They are referred to here as 1:1 path switching, 1+1 pathswitching, and 1:1 line switching. FIG. 1 a illustrates 1+1 pathswitching. The ring on the right demonstrates a protection switch. Thismethod duplicates traffic entering the ring and dual-feeds it along botha working path and a protection path. The destination node chooses apath based on path status information. In SONET, a path is an STS or aVT. In ATM, a path can be a VP group, a VP or a VC.

FIG. 1 b illustrates 1:1 path switching wherein the dotted linerepresents the protection path. The source node transmits traffic alongthe working path only. When a fault is detected in the ring, failuremessages are propagated to the source nodes of all affected paths. Thesource nodes switch the working paths to the protection paths travelingthe opposite direction around the ring.

The third method, 1:1 line switching, does not switch traffic on anindividual path basis; rather, the node upstream of the failure reroutesall traffic in a bundled fashion away from the failure. FIG. 1 cillustrates one embodiment of a switching method where the destinationnodes receive connections from either link. This method, commonlyreferred to as Kajiyama's line method, uses only one loopback forswitched traffic. The line switching mechanism in SONET rings results ina double loopback because a particular connection can only be receivedfrom one link.

These protection methods work for unidirectional or bidirectional rings.In a unidirectional ring all working traffic travels the same directionaround the ring, and all protection traffic travels the oppositedirection. Thus, working traffic is dedicated to one fiber, and twopaths of a duplex connection contain a disjoint set of intermediatenodes. In bidirectional rings, working traffic may be assigned to fibersin both directions. In general, each direction of a duplex connectiontraverses the same ring nodes but on different fibers.

It is possible to develop expressions for the required capacity for thering size needed to support a particular traffic demand. In developingsuch expressions, all links on all fibers of a ring are assumed to havethe same link rate; e.g., OC-12, etc. The size of the ring, or,similarly, the amount of traffic that can be placed on the ring, isdetermined by the required capacity for a particular set of connections.The required capacity is given by the maximum of the minimum bandwidthneeded on any link to support a particular traffic pattern under anon-failure or any single location failure scenario. The bandwidth ofthis link, the maximum bandwidth link, gives the required capacity orminimum ring size needed to fully protect the traffic. The requiredcapacity depends upon the traffic demand pattern, the protection scheme,the routing method and the spare capacity allocation method.

As mentioned above, the ring may be unidirectional or bidirectional. Thering type, which is determined by the routing method, impacts therequired capacity. Another factor which impacts the required capacity iswhether the demand between node pairs is symmetric or asymmetric. Ananalysis for the required capacity for the three protection schemes onunidirectional and bidirectional rings with symmetric and asymmetricdemand.

In 1+1 path switching there is no routing choice since both paths fromsource to destination are active. The 1+1 path switching scheme isconsidered a unidirectional ring. (Typically, the default working pathsare designated to one particular fiber.) The working paths may beassigned in a bidirectional sense where the working paths of a duplexconnection traverse the same nodes but on opposite ring fibers; however,this distinction between working and protection paths does not affectthe required capacity. For symmetric duplex connections, the dual-fedproperty of this protection scheme causes one direction of the demandbetween node pairs to be present on each link of the ring. Thus, if d(i,j) represents the one-way demand bandwidth between node pairs i and j,the required capacity, RC, is given by:

$\begin{matrix}{{RC} = {\sum\limits_{{one}\mspace{14mu} {way}}{d\left( {i,j} \right)}}} & (1)\end{matrix}$

For asymmetric connections, the bandwidth demand on each link may vary.The ring capacity is given by the link with the maximum bandwidth. Asimple analysis of the 1+1 path switched ring indicates that simplexdemand combinations on one fiber may require more bandwidth thancombinations on the other fiber. This is illustrated in FIG. 2. The thinline represents a connection requiring d bandwidth. The thick linerepresents a connection requiring d+m bandwidth. Although the extrabandwidth m is available between nodes 1 and 4 on the outer fiber, usingthis bandwidth for a connection other than between nodes 1 and 4 resultsin an overlap on the inner fiber of the new connection with the d+mconnection. The dotted line in FIG. 2 represents the new connection. Therequired capacity is d+2m as defined by the inner links between nodes 2and 4.

The 1:1 protection methods are suitable for both unidirectional andbidirectional rings. Because these methods have only one active pathbetween the source and destination, a routing choice exists for eachconnection. In 1:1 path and 1:1 line switching in a unidirectional ring,the working fiber contains the same topology as the working fiber in 1+1path switching. The only difference is that the protection fibercontains no traffic; it contains only the traffic from the failed spanafter the protection switch. (This difference may be significant becausethe bandwidth is available for use by a non-protected class of traffic.)Thus, for symmetric connections on a unidirectional ring, the requiredcapacity for the 1:1 protection schemes is given by equation (1) above.

For asymmetric connections, however, a bandwidth advantage may exist forthe 1:1 protection methods over 1+1 path switching. As shown in FIG. 2,the outer (working) fiber needs d+m bandwidth to support theconnections. This working fiber determines the required capacity for 1:1protection, whereas 1+1 path switching requires d+2m capacity on theinner fiber. Thus, on average, the 1:1 protection methods inunidirectional rings require equal or less capacity than 1+1 pathprotection for all demand patterns.

Bidirectional rings, such as SONET rings, may contain either 2 or 4fibers. Four-fiber rings reserve a fiber in each direction forprotection traffic. Two-fiber bidirectional rings reserve a portion ofthe bandwidth on each fiber for protection traffic. Existing 2-fiberrings reserve 50% of the bandwidth on each link for protection traffic.This spare bandwidth allocation factors into the ring capacitycalculation. The link containing the maximum working traffic bandwidthfor a given demand pattern is used as the maximum bandwidth link. Thering capacity is given by twice this bandwidth. The ring capacity forany demand pattern is the same for 1:1 path switching and 1:1 lineswitching even though the two methods produce different protection pathsfor the same working traffic. The working traffic alone determines thering capacity because there is always enough spare bandwidth to reroutethe maximum bandwidth link's working traffic.

The present inventors have recognized that each bidirectional ring with1:1 path protection may be designed using minimum spare bandwidthallocation methods to produce the smallest ring capacity requirement forany traffic demand pattern. Such minimum spare bandwidth methods may beadapted for use in a connection admission controlled method for minimumspare bandwidth allocation. To this end, the present inventors haveexamined the required ring size for three self-healing mechanisms undersymmetric and asymmetric demand and different routing schemes. They haveshown that asymmetric connections adversely affect 1+1 path switchingwhen compared to 1:1 protection switching on unidirectional rings. Thisin turn allows the 1:1 methods to require a smaller unidirectional ringsize. Similarly, their analysis of past bandwidth allocation methods onbidirectional rings which reserve 50% of the total bandwidth on eachspan for protection traffic show that the relative capacity of theprotection methods are dependent on the demand pattern. The proposedminimum spare bandwidth assignments for the 1:1 protection methods,however, decrease the required capacity of bidirectional rings for bothsymmetric and asymmetric traffic. This optimal partitioning of workingand protection bandwidth makes a bidirectional ring with 1:1 pathswitching the most bandwidth efficient method for all demand patterns.

BRIEF SUMMARY OF THE INVENTION

The present invention provides for a method for reserving sparebandwidth for a link in a communication network including a plurality oflinks. The method provides for monitoring the volume of traffic routedthrough each link of the communication network. A single link failurefor each link is then simulated and the volume of traffic which would bererouted through each link for maintaining communication and the volumeof traffic removed from each link are determined for each simulatedsingle link failure. The difference between the volume of traffic whichwould need to be rerouted through each link and the corresponding volumeof traffic removed from each link is then computed, and a maximumdifference value is determined for each link for all simulated singlelink failures. An amount of spare bandwidth equivalent to the determinedmaximum difference is then reserved for each link.

In an alternative embodiment a connection admission control method foruse in a communication network is provided. The method provides for thereceipt of a request for a communication connection. The bandwidthneeded for the requested communication and the maximum additional sparebandwidth needed is determined and the sum of the two is compared to theavailable bandwidth for each of the links. If sufficient bandwidth foreach of the links is available, the communication request is accepted.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is an illustration of several different SHR ring networktopologies.

FIG. 2 illustrates asymmetric connections on a 1+1 path switched ring.

FIG. 3 illustrates bandwidth assignments in a 1:1 protection ring.

FIGS. 4-11 are graphs comparing the required capacity for various SHRprotection methods.

FIG. 12 illustrates an example of point-to-multipoint communication viaa self healing ring network topology.

DETAILED DESCRIPTION OF THE INVENTION

With the appropriate bandwidth granularity, it is possible to allocatespare bandwidth only as needed on each link of a bidirectional ringrather than reserving 50% of the bandwidth on each span for protectiontraffic. An optimal spare bandwidth allocation reserves the minimumbandwidth needed on each link such that the worst case failure issupported. This allocation, in turn, means the required capacity isgiven by the link with the maximum bandwidth after any failure ornon-failure scenario. The maximum bandwidth link is now defined by boththe working and protection traffic. The worst case failure scenario is alink failure if all traffic destined for a failed node is removed fromthe ring. (This analysis also assumes that a link failure causes workingtraffic on both links of the span to undergo protection switching.) FIG.3 shows an example of the protection bandwidth assignments for aparticular demand pattern on a four node ring with 1:1 protection. Inthis example each connection requires the same bandwidth d and the ringsize is given by 3d.

Unlike the 50% method, the required capacity with minimum sparebandwidth may differ for the 1:1 protection methods. Now, the requiredcapacity for 1:1 path switching is always less than or equal to thatrequired by 1:1 line switching and 1+1 path switching. In 1+1 pathswitching traffic from all protection paths is present on the ring,where in 1:1 path switching, only traffic from the failed link ispresent. In 1:1 line switching the protection connections loopback fromthe point of failure to the source node on the opposite ring fiber, andthen proceed to the destination node. (See FIG. 1 c.) The protectionpaths for 1:1 path switching do not include the looped back portion ofthe protection path, resulting in the same or less traffic on each link.

Another observation is that the required capacity for 1:1 path switchingon bidirectional rings with minimum spare bandwidth is independent ofthe routing scheme. The independence results from the fact that after asingle location failure, 1:1 path switching produces the same topologyregardless of the original routing scheme. The failure location dictateswhich direction the connection must traverse the ring. Thus, providedthat the working paths alone do not yield a higher maximum bandwidthlink than the working plus protection paths (as in unidirectionalrouting), the required capacity for 1:1 path switching is independent ofrouting.

Expressions for the required capacity in a ring are possible givenknowledge of the connections and their associated bandwidth. Theseexpressions are developed below for a 2-fiber ring, although they can beextended to 4-fiber rings by allowing links to accommodate both workingand protection traffic. Each span connecting adjacent nodes of a 2-fiberring has two links, one carrying traffic in the clockwise direction andone in the counter-clockwise direction. Let i and j represent twodifferent spans on a ring, and let C(i, 0) denote the set of workingconnections and their associated bandwidth traversing span i in theclockwise direction, and let C(i, 1) denote the set of workingconnections on span i in the counter-clockwise link. After a failure atspan j, the working and protection connections on the clockwise link ofspan i, C_(wp)(i, j, 0) is represented by the equation:

C _(wp)(i,j,0)=C(i,0)+C(j,1)−(C(i,0)∩C(j,0))−(C(i,1)∩C(j,1))  (2)

where C(i, 0)∩C(j, 0) represents the working connections removed fromthe clockwise link on span i by the failure and C(i, 1)∩C(j, 1)represents connections on the counter-clockwise link of span j which arenot rerouted across the clockwise span i link. Likewise, the working andprotection connections on the counter-clockwise link of span i after afailure at span j is given by:

C _(wp)(i,j,1)=C(i,1)+C(j,0)−(C(i,1)∩C(j,1))−(C(i,0)∩C(j,0))

The required capacity for asymmetric connections is given by the maximumof equations (2) and (3) for all i, j. If the connections are symmetric,equations (2) and (3) are equal, and the required capacity is given bymaximizing equation (2).

The connection topology of 1:1 line switching after a failure is givenby the connections of 1:1 path switching plus the looped back portionsof the protection connections. Looping back may result in less elementsin the C(i, 0)∩C(j, 0) and C(i, 1)∩C(i, 1) sets of equations (2) and (3)depending on whether the failure is upstream or downstream of i for aparticular connection. Thus, 1:1 line switching requires the same ormore ring capacity than 1:1 path switching. Moreover, the ring capacityfor 1:1 line switching is dependent upon the routing method. The routingmethod determines the length of the looped back segments of theprotection connections.

The independence with respect to routing inherent in 1:1 path switchingmeans that only knowledge of the total demand between node pairs, notknowledge of the individual connection paths, is needed. Thus, if D(i,j, 0) and D(i, j, 1) represent the set of demand present on link i inthe clockwise and counter clockwise direction, respectively, given afailure on link j; the ring capacity is given by the maximum of D(i, j,0) and D(i, j, 1) for all i and j. As is shown below, a connectionadmission control or a dynamic bandwidth allocation method can use thisprocess for a 1:1 path switching ring.

Exemplary simulations and analysis results for the required capacity ofvarious protection schemes with the fixed and minimum spare bandwidthallocation were executed by the present inventors. The exact logicaltopology of the demand pattern on the ring is application dependent;however, two common topologies are the centralized and the fullydistributed, or mesh, demand pattern. In the centralized demand patternall ring nodes are connected to a central, or hub, node only. No directconnections exist between non-hub nodes. The fully distributed topologysupports connections between every node pair on the ring. In addition,duplex connections may be symmetric or asymmetric. The analysis assumesthat all demand between node pairs is carried along the same path (nodemand splitting).

The required capacity is easily analyzed for centralized and mesh demandpatterns when the demand between connected node pairs is equal andsymmetric. Equal symmetric demand is common in WDM optical rings. Asmentioned above, all three protection schemes on unidirectional ringsyield a required capacity for symmetric connections given by the sum ofall the oneway demand on the ring. Thus, if the demand between connectednode pairs is d, the ring capacity for a centralized demand is given byRC=(n−1)d For a mesh demand, the capacity is RC=(n(n−1)d)/2. This demandis present on all links for 1+1 path switching, but on only the workingfiber for the 1:1 methods.

The required capacity of the 1:1 methods for equal symmetric demand onbidirectional rings is dependent on the spare bandwidth allocation. Forthe 50% method, shortest path routing gives the minimum maximumbandwidth link for the centralized and mesh demand patterns with equalsymmetric connections. The ring capacity for both 1:1 path and 1:1 lineswitching with 50% spare bandwidth and a centralized demand pattern isgiven by:

$\begin{matrix}{{RC} = \left\{ \begin{matrix}{\left( {n - 1} \right)d} & {{odd}\mspace{14mu} n} \\{nd} & {{even}\mspace{14mu} n}\end{matrix} \right.} & (4)\end{matrix}$

Note that this RC is larger than on a unidirectional ring for even n.Thus, for equal symmetric centralized demand, the protection schemeyielding the minimum required capacity is a function of the routingmethod.

With a mesh demand topology, 1:1 protection with bidirectional routingmakes more efficient use of ring bandwidth than 1+1 path protection. Forodd n, each link contains (n²−1)/8 working connections with shortestpath routing. When n is even, the maximum number of working connectionson a link is given by [n²/8]+1, where [X] is the largest integer ≦X.Thus, the ring capacity for the 50% method is:

$\begin{matrix}{{RC} = \left\{ \begin{matrix}\frac{\left( {n^{2} - 1} \right)d}{4} & {{odd}\mspace{14mu} n} \\{\left( {\left\lfloor \frac{n^{2}}{8} \right\rfloor + 1} \right) \times 2d} & {{even}\mspace{14mu} n}\end{matrix} \right.} & (5)\end{matrix}$

As n increases, this required capacity approaches only half the amountneeded for 1+1 path switching. Note that with the 50% method, therequired capacity for 1:1 path switching and 1:1 line switching is thesame.

For minimum spare bandwidth allocation, the ring capacity is given bythe link with the maximum working and protection bandwidth after anyfailure or non-failure scenario. The required capacity for bidirectionalrings with even n and equal symmetric demand decreases under this sparecapacity allocation. For example, in a equal symmetric centralizeddemand pattern, 1:1 protection yields a ring capacity of RC=(n−1)d whenn is even. (See FIG. 3.) Similarly, a mesh demand pattern has a ringcapacity of

${RC} = \left\{ \begin{matrix}\frac{\left( {n^{2} - 1} \right)d}{4} & {{if}\mspace{14mu} n\mspace{14mu} {is}\mspace{14mu} {odd}} \\\frac{n^{2}d}{4} & {{if}\mspace{14mu} n\mspace{14mu} {is}\mspace{14mu} {even}}\end{matrix} \right.$

The reduction in ring size is most pronounced for n=4 (67%). Both 1:1path switching and 1:1 line switching yield the same ring capacity forshortest path routing. In 1:1 line switching with shortest path routing,the looped back sections of the protection paths lie within n/2 nodes ofthe failed span. Although 1:1 path switching has a capacity advantageover 1:1 line switching on this portion of the ring, the difference doesnot affect the required capacity.

Random symmetric demand exists when the connections between node pairsare symmetric, but the demand between different node pairs varies. Thistraffic pattern exists on currently deployed SONET rings. As mentionedabove, all protection methods on unidirectional rings have the samecapacity for random symmetric demand. Differences in required capacityexist, however, for bidirectional rings. The ring capacity forbidirectional rings with 50% spare bandwidth is the same for both 1:1path and 1:1 line switching, but differs from 1+1 path switching.Analysis on bidirectional rings is more difficult with these trafficpatterns, so the average bandwidth advantage of minimum spare capacitymay be examined through simulations.

For simplicity, the simulations create one connection between node pairsin a given topology, and this connection defines the total node-pairdemand. The connections are assigned bandwidth randomly with a uniformdistribution. Two different bidirectional routing schemes are used forthe working traffic. One routing method is shortest path routing (SP)where the connection is established in the direction containing theleast hops. Shortest path routing completely determines the direction ofthe connections when n is odd. When n is even, the direction chosen forconnections which traverse n/2 nodes alternates for adjacent n/2 lengthconnections. This method assumes a prior knowledge of all connections onthe ring. The method first routes all connections according to theshortest path. Then, the longest path for each connection is tested, inan iterative fashion, to see if the maximum working bandwidth link isreduced. The direction for the connection yielding the minimum maximumbandwidth link is chosen. This iteration is repeated for allconnections. This method is guaranteed to yield the same or lessbandwidth for the maximum bandwidth link when compared to the shortestpath method. Routing methods attempting to minimize the maximumbandwidth link on a first come, first serve basis, without a priorknowledge of all connections, are not considered for these simulationsbecause, on average, they yield a larger maximum bandwidth link thanshortest path routing.

The mesh topology is discussed first. The demand between node pairs issymmetric but randomly chosen with a uniform distribution for each nodepair. For comparison, the required capacity for the 1:1 methods dividedby the required capacity for 1+1 path switching is used as a relativecapacity measure. FIG. 4 illustrates the average relative capacity withrandom symmetric demand and 50% spare capacity allocation. Also shown inthe figure is the relative required capacity under equal symmetricdemand: that is, equation (5) with d equal to the average demandbandwidth. On average, the 1:1 methods with random symmetric demandrequire less bandwidth than 1+1 path switching in a mesh topology. Theonly exception was for the SP method with n=4. The RSP routing methodreduced the required capacity over the SP routing method by an averageof 4%. However, the bandwidth advantage of the 1:1 protection methodsover 1+1 path switching for random symmetric demand is not as great asfor equal symmetric demand. The variance of the working bandwidth onindividual links is greater than the variance of the total demand,thereby decreasing the average difference between the total bandwidthand the maximum bandwidth link.

The average relative capacity with minimum spare bandwidth in a randomsymmetric mesh topology is shown in FIG. 5. Also shown is equation (6)with d equal to the average demand bandwidth. As expected, the 1:1 pathswitching method requires less capacity than the 1:1 line switchingmethod, although the difference is slight. The SP routing methodrequires less capacity for 1:1 line switching than the RSP method. Thisis not surprising since some of the demand in the RSP traverses thelongest path. This in turn increases the length of the loopback sectionof the line switched protection path, causing the connection to bepresent on more links.

To understand the advantage of minimum spare bandwidth over fixed 50%reservation, the required capacities yielded for FIG. 4 are compared tothose shown in FIG. 5. FIG. 6 demonstrates the percentage of requiredcapacity needed with minimum spare bandwidth allocation and randomsymmetric mesh demand compared to the 50% method. The bandwidthadvantage is greatest for small n. When n=4, the minimum spare bandwidthassignment for 1:1 path switching and SP routing requires only 73% ofthe capacity necessitated by the 50% method. This bandwidth advantagediminishes for increasing n. As n increases the link bandwidth variancedecreases, causing the minimum spare bandwidth method to require closeto 50% spare bandwidth for each link. The difference with the RSProuting method is less due to the reduced ring capacity in the 50%method. (1:1 path switching with minimum spare bandwidth is unaffectedby the routing method.)

FIG. 7 illustrates the average relative capacity with random symmetriccentralized demand and 50% spare bandwidth. The 1:1 protection methodsrequire a larger ring size for the same node pair demand pattern.Consistent with equation (4), the difference is most pronounced forsmall n and for even n. At n=4, 1:1 protection with SP routing requires34% more capacity than 1+1 protection. This tapered off to 10% morecapacity for n=20. RSP routing reduced the relative capacity by anaverage of 8%. A small n results in a greater variance in link bandwidthwhich yields a larger average maximum bandwidth link relative to thetotal bandwidth.

The required capacity of 1:1 path switching with minimum sparebandwidth, resulting from maximizing equations (2) and (3) is simplifiedfor centralized demand. A failure scenario with 1:1 path switchingcauses each link to contain only one type of traffic: namely,connections with the hub node as the source, C(from hub), or with thehub node as the destination, C(to hub). If SP routing is used, the sameis true for a non-failure scenario. The required capacity for 1:1 pathprotection and centralized demand is:

RC=max{C(to hub),C(from hub)}  (7)

For symmetric connections, however, C(to hub)=C(from hub) 1:1 lineswitching with bidirectional routing cannot guarantee the segregation oftraffic on each link according to the direction of flow in relation tothe hub node. This results from the loopback portions of the protectionpaths during failure. With shortest path routing, however, the linkscontaining a mix of “to hub” and “from hub” traffic do not contain moreconnections than links adjacent to the hub node. The symmetric nature ofthe connections will thus cause these “mixed” links to contain equal orless bandwidth than the links adjacent to the hub node. Thus, like 1:1path switching, the ring capacity is given by the sum of all one-waydemand on the ring. Therefore, 1+1 path protection and the 1:1protection methods on bidirectional rings with minimum spare bandwidthand 1+1 path protection yield the same required capacity for symmetriccentralized demand. Thus, the bandwidth advantage of minimum sparecapacity is given by the inverse of the plots in FIG. 7.

Random asymmetric demand occurs when the simplex demand between nodepairs is allowed to vary. This traffic pattern is likely to exist infuture connection oriented rings with ATM layer protection schemes orwith asymmetric SONET connections. The mesh topology is discussed first.The average relative required capacity for random asymmetric mesh demandwith shortest path routing is shown in FIG. 8. Also shown in this figureis equation (6) with the d set to the average simplex demand. Theability to reuse protection bandwidth inherent in 1:1 protection onbidirectional rings gives a capacity advantage over 1+1 path switching.Note from FIG. 8 that 1:1 line switching requires a slightly larger ringsize than 1:1 path switching for shortest path routing. Again, theshorter loopback sections created in shortest path routing produced aslight capacity advantage over the RSP method. FIG. 8 shows that therelative ring size required by 1:1 path switching with minimum sparebandwidth and random asymmetric demand is more than the equal symmetricdemand case. However, comparing FIG. 8 with FIG. 5 shows that the 1:1protection with asymmetric demand gives a greater reduction in requiredcapacity from 1+1 path switching than for symmetric demand. Thereduction is caused by the increase in capacity found in 1+1 pathswitched rings with asymmetric traffic as shown in FIG. 2.

To understand the advantage of minimum spare bandwidth, the ring sizesyielded for FIG. 8 are compared to those given by the 50% method. FIG. 9demonstrates this difference for 1:1 path switching and randomasymmetric mesh demand. The bandwidth advantage for both routing schemesis greatest for small n. When n=4, the minimum spare capacity assignmentfor 1:1 path switching requires 73% of the required capacitynecessitated by the 50% method. This bandwidth advantage diminishes forincreasing n because of the reduced variance in link bandwidth. Theadvantage for 1:1 line switching is similar, although slightly less.

FIG. 10 and FIG. 11 illustrate simulation results for random asymmetriccentralized demand. FIG. 10 compares the average required capacity ofthe 1:1 protection with 50% spare bandwidth to 1+1 path switching. Thisfigure shows that 1:1 protection with shortest path routing requires alarger ring size that 1+1 path switching. As expected, the difference isless for asymmetric demand than for symmetric demand. (See FIG. 7.) Aresult is that the RSP routing method produces a smaller averagerequired capacity for 1:1 protection than for 1+1 path switching forn>10.

FIG. 11 gives the same comparison as FIG. 10, but for minimum sparebandwidth allocation. It is noted that minimum spare bandwidth causesthe 1:1 protection methods to require less capacity than 1+1 pathswitching for any n, with the decrease most pronounced for small n. Forn=5, 1:1 path switching requires an average of 91% of the bandwidthneeded by 1+1 path switching. In unidirectional routing, links adjacentto the hub node contain only one kind of traffic, but other links maycontain a mix of to hub and from hub traffic. Thus, 1:1 path switchingshould require less capacity than 1+1 path switching for randomasymmetric centralized demand. Likewise, 1:1 line switching cannotguarantee the segregation of traffic on each link according to thedirection of flow in relation to the hub node. Unlike symmetric demand,however, asymmetric demand can cause these “mixed” links to contain moretraffic than the total “to hub” or “from hub” traffic. Thus, it ispossible for these mixed links to cause the required capacity of 1:1line switching to be greater than 1:1 path switching. This difference isseen in FIG. 11 as well.

In summary of the relative required capacity for the three protectionschemes, it has been found that the difference in capacity between theseprotection methods is dependent upon the routing method, the demandpattern and the spare bandwidth allocation. For a unidirectional ring,all protection methods perform the same for symmetric duplex demand.Asymmetric connections adversely affect 1+1 path switching when comparedto 1:1 protection, causing the 1:1 protection methods on unidirectionalrings to yield a smaller required capacity. For bidirectional rings with50% spare bandwidth, the protection scheme yielding the minimum ringcapacity is a function of the demand pattern. For random symmetriccentralized demand, the 1:1 methods require more capacity than 1+1 pathswitching. For random asymmetric centralized demand, 1:1 protectionmethods sometimes require less capacity than 1+1 path switching. Therelative capacity depends on the routing method and the number of nodesin the ring. For a random mesh demand pattern, the 1:1 methods requireless capacity that 1+1 path switching. The difference is most pronouncedfor asymmetric demand.

When minimum spare bandwidth is used on bidirectional rings, 1:1 pathprotection requires the same or less capacity than the other methods forall demand patterns. The foregoing analysis and simulations show that,in comparison to the 50% method, minimum spare bandwidth decreases therequired capacity for both a centralized and a mesh demand pattern witheither symmetric or asymmetric connections. For random symmetriccentralized demand, the 1:1 protection methods perform the same as 1+1path switching. For random asymmetric centralized demand, the 1:1protection methods use approximately 92% of the capacity needed by 1+1path switching. Moreover, 1:1 path switching uses slightly less capacitythan 1:1 line switching and is independent of the routing method. For arandom symmetric or asymmetric mesh demand, minimum spare bandwidthmakes 1:1 path switching the most bandwidth efficient method. 1:1 lineswitching with shortest path routing follows closely. In some cases, theminimum spare bandwidth allocation reduces the required capacity by 30%over the 50% method.

In view if the foregoing, a connection admission control (CAC) methodfor 1:1 path switching with minimum spare bandwidth in a bidirectionalring has been developed. The CAC method for the 50% method need onlyexamine the working bandwidth on each link. The CAC method for minimumspare bandwidth, however, should account for the working and protectiontraffic present on a link after any single location failure. Theanalysis presented above is implemented in the newly developed CAC usingrequired capacity states for each link. Because a duplex connection maycontain an asymmetric bandwidth assignment, the CAC method is assumed toexamine each simplex part of the duplex connection separately. The CACmethod disclosed herein determines if enough bandwidth exists to supporta simplex connection. The decision to admit a duplex connection involvesto outcome of both simplex paths.

A new simplex connection may traverse one of two paths depending on thefailure location. (See figure FIG. 1 b.) Together, these two paths coverall n spans in the ring, although only one link is used on each span.(One path uses the inner ring, while the other path uses the outerring.) Of these two paths one path is the working path, i.e, this pathis active in a non-failure scenario. Let R(i, j) represent the requiredcapacity needed on a link of span i given a failure at span j. Sincethere are a total of n spans, n state variables exist for each link. TheR(i, i) state is not needed because no traffic exists on a failed link;however, a state is needed to represent a non-failure condition. LetR(i, i) define the working traffic on a link of span i. Thesedefinitions are used in the update of the R(i, j) states:

-   -   S The set of spans on a ring.    -   P(c) The set of spans traversed by connection c.    -   P(c)-bar The set of spans not traversed by connection c.    -   b(c) The bandwidth associated with connection c.

L Total bandwidth on link.

For each link in the two possible paths of a simplex connection, thestates are updated according, to the following equation:

R(i,i)=R(i,i)+b(c) for c=working connection

R(i,j)=R(i,j)+b(c) for {iεP(c),jε P(c)}  (8)

That is, the connection bandwidth is added to the non-failure state ifit is a working connection, and it is added to the states representingfailures on spans traversed by the opposite path. A failure on any ofthe P(c) spans will remove connection c from the link; thus, thesestates are not updated. A connection is accepted if, after the update inequation (8),

R(i,j)<L for all i,j  (9)

A connection is rejected if this condition is not met. Likewise, when aconnection is removed from the ring, the capacity states for theappropriate links are updated according to equation (10):

R(i,i)=R(i,i)−b(c) for c=working connection

R(i,j)=R(i,j)−b(c) for {iεP(c),jε P(c)}  (10)

If it is desired to allow 1:1 path protection to be non-revertible, theR(i, i) state is updated after a failure by setting R(i, i)=R(i, j) fora failure at j.

The CAC method for minimum spare bandwidth can be implemented in acentralized or distributed fashion at the systems, such as add dropmultiplexers (ADMs), disposed at the ring nodes. Each node should haveknowledge of all spans traversed by a simplex connection. To this end,the CAC can be implemented in a distributive manner if this informationis included in a call set-up message, either by including a list ofthese spans in the message or by including the source and destinationnode and having each node calculate the traversed spans.

There are n total links traversed by the two paths. The working pathcontains n state updates (addition or subtraction), and the protectionpath contains n−1 state updates. Thus, the number of state updates is onthe order of W. In addition, the total number of capacity states is nfor each link or n² for the whole network.

The foregoing principles for point-to-point connections can be extendedto other types of ring networks and other types of communicationconnections. As a further example point-to-multipoint connections areillustrated. For point-to-multipoint connections, data from one sourcenode is sent to multiple destination nodes. These connections savebandwidth over separate point-to-point connections.

For the sake of bandwidth efficiency, point-to-multipoint connectionsare “split” onto both fibers such that the source node connects to somedestination nodes along one fiber while reaching other nodes on theopposite fiber. For these split connections, the source node dual-feedstraffic along both fibers. The destination nodes upstream from the finaldestination node on a particular path employ a drop and continuefunction. A point-to-multipoint connection beginning at source node S,and broadcasting to all other nodes, is shown by the solid lines in FIG.12 a.

The protection paths for the point-to-multipoint connection are shown bythe dashed lines in FIG. 12 a. The protection paths extend from the lastdestination node on the working path to the destination node prior tothe source node. The dots in this figure indicate drop points on thepaths. If nodes adjacent to a failure do not send and/or receive trafficacross the failed section, protection switching is accomplished by onenode. For a failure on one path of the dual-fed path, the lastdestination node on the opposite path switches to the protection path.This protection switch results in the “extending” of the path to thefailure location as shown in FIG. 12 b. Thus, if both links of a spanare considered failed, both parts of the split paths beyond the failure(one working, one protection) are removed. Note that if the demand isnot split onto both fibers, the source node accomplishes protectionswitching by turning on the dual-feed function. As can be seen in FIG.12 b, after the link failure only a portion of the traffic was reroutedby extending from the last destination node via the protection path. Ascan be seen by this example a reserve capacity of less than 50% is oftenadequate.

By way of further example, a preferred embodiment of the connectionadmission control (CAC) method extended to point-to-multipointconnections is illustrated below.

Specifically, for multipoint traffic, shortest path routing puts theworking path on both the clockwise and counterclockwise rings. Denotew_(d)(c) as the set of links forming the working path on the d directionring, and w_(d)*(c) as the working path on the other ring. Similarly,let p_(d)(c) be the part of the protection path on the d direction ring,and let p_(d)*(c) be the other part of the protection path on the otherring.

Also, let w_(d)(c,i_(d)) be the contiguous set of links starting fromthe source up to i_(d), inclusively, traversing in the d direction. Theconnection admission control method comprises the following analysis:

-   -   Step 1: If any of the following two conditions are true:    -   1) i_(d)εw_(d)(c) and j∩w_(d)(c,i_(d))=        -   (i_(d) is part of the working path, and a failure at j does            not affect the working path)    -   2) i_(d)εp_(d)(c) and j∩{w_(d)*(c,i_(d)*)−i_(d)*}·        -   (i_(d) is part of the protection path, and i_(d) will be            used by c if j fails.)    -   then        -   R′(i_(d),j)=R(i_(d),j)+b(c)    -   Otherwise,        -   R′(i_(d),j)=R(i_(d),j)    -   Step 2: If R′(i_(d), j) is less than the ring capacity for all        i_(d) and j, then admit c, and update R′(i_(d),j)→R(i_(d),j).

To update R(i_(d),j) when a call terminates, letR(i_(d),j)←R(i_(d),j)−b(c) for all R(i_(d),j) satisfying the conditionsin step 1.

The connection admission control method ensures enough capacity tohandle all admitted traffic under any single location failure scenario.

The connection admission control method described forpoint-to-multipoint traffic is simple in complexity. Furthermore theconnection admission control method can be readily implemented using astate table, which is updated to represent the required bandwidth underall failure scenarios. The simplicity makes minimum spare bandwidth aneffective bandwidth allocation method for producing an efficientself-healing ring.

Numerous modifications may be made to the foregoing system withoutdeparting from the basic teachings thereof. Although the presentinvention has been described in substantial detail with reference to oneor more specific embodiments, those of skill in the art will recognizethat changes may be made thereto without departing from the scope andspirit of the invention.

1.-19. (canceled)
 20. A connection control method for a ring-typecommunication system, the method comprising: evaluating bandwidthrequired for a proposed connection; evaluating bandwidth required forworking and protection traffic on the ring in at least one failuresituation; accepting the proposed connection only in the presence ofadequate excess capacity to support same in the at least one failuresituation.
 21. A method as in claim 20 which includes adjustingbandwidth requirements when a connection is dropped.
 22. A method as inclaim 21 which includes conducting the evaluating on a distributedbasis.
 23. A ring-type communication system comprising: a working pathand a protection path; at least one node coupled to the paths; apparatusfor accepting a proposed connection only in the presence of adequatebandwidth notwithstanding at least one failure on one of the paths. 24.A system as in claim 23 where the apparatus comprises, at least in part,an add drop multiplexer at the at least one node.
 25. A system as inclaim 23 which includes a plurality of nodes each of which contains anadd drop multiplexer where the apparatus is distributed there among. 26.A device for a communication system comprising: at least a first pair ofports for ingress and egress of communications signals from respectivelinks in the system; add drop processing structures coupled to the portsfor adding and dropping communications associated with the links; and aconnection evaluator coupled to the structures which, at least in part,evaluates bandwidth requirements, if a requested connection is acceptedand if a link fails.
 27. A device as in claim 26 which includes firstand second pairs of ports coupled to the processing structures forbi-directional communications.
 28. A device as in claim 26 where theevaluator compares the bandwidth requirements to available linkbandwidths.
 29. A device as in claim 26 which includes communicationsrelated data used by the evaluator in determining if a requestedconnection should be accepted.
 30. A device as in claim 28 whichincludes a plurality of displaced processing structures and connectionevaluators where at least some of the processing structures include adddrop multiplexers.
 31. A device as in claim 30 where the displacedprocessing structures are linked, via ports and communications links,the links at least during selected time intervals carry substantiallyduplicate communications information routed in opposite directions. 32.A device as in claim 26 where the processing structures comprise a nodeof an optical communication system.
 33. A device as in claim 31 wherethe displaced processing structures each comprise a four port node of anoptical communications system.